1. Field of the Invention
This invention relates to an infrared sensor for the non-contact measurement of the temperature of a given object and a method for the production thereof. More particularly, it relates to an infrared sensor provided on an infrared temperature-sensing film with an electrode and a method for the production thereof.
2. Description of the Prior Art
Recently, various techniques have been developed for the production of infrared sensors usable ideally for non-contact thermometers, fop example, by effective use of the technique of microfine fabrication of semiconductors. The infrared sensors come in various types such as for example, those of the thermobolometer type and the thermopile type. The infrared sensors of the thermobolometer type are superior to those of other types in allowing easy dimensional reduction because of the fact that their sensitive film which is made of a substance fulfilling the function of a thermistor, is capable of measuring an increase in temperature caused by the incidence of infrared radiation by measuring a change in the magnitude of electrical resistance and consequently determining the amount of the infrared radiation.
In the infrared sensor of the thermobolometer type, if the infrared temperature-sensitive film of the sensor has a small heat capacity, the amount of heat transmitted outwardly from the film decreases, the temperature rises remarkably and the response of the sensor to a minute amount of infrared radiation gains in sensitivity in proportion. The conventional infrared sensor of the thermobolometer type, therefore, has been constructed so that an extremely small bridging part is formed on a supporting substrate made of a semiconducting material and an infrared sensitive film is formed on the bridging part by the microfine fabrication technique. To be specific, the bridging structure keeps a temperature-sensing part of the sensor afloat from the supporting substrate for the purpose of improving the response sensitivity of the sensor.
The infrared sensor requires various contrivances for the sake of measurement because it is intended to handle a very minute amount of infrared radiation emitted from a given object. For example, one infrared sensor is provided with two entirely identical infrared temperature-sensitive films having bridging structures; one of the films is exposed to the incidence of infrared radiation and the other shielded from the infrared radiation. By constantly comparing the output of the infrared temperature-sensitive film exposed to the infrared radiation and the output of the infrared temperature-sensitive film shielded from the infrared radiation, the infrared sensor is enabled to exclude the electrical noise and thermal disturbance and detect the net amount of infrared radiation.
The infrared sensor of this type is furnished with various contrivances so as to ensure an efficient incidence of infrared radiation on the exposed infrared sensitive film. From the practical point of view, however, the infrared sensor requires the minute bridging structure formed therein to be protected mechanically and physically. Heretofore, for the protection of the bridging structure, the practice of keeping the infrared sensor device in such a container as a can has been commonly used.
The conventional infrared sensor has the sensor element thereof kept in a container as described above. When the sensor accommodated in the container as described above is used as a non-contact thermometer, it poses problems in relation to thermal conductivity, restriction in size, and cost of production. Thus, there is a need for a packaging method which allows the sensor element to be protected as required by a simple procedure at a low cost.
In the infrared sensor of this kind, for the purpose of ensuring selective incidence of the infrared radiation on one of the pair of infrared temperature-sensitive films, the sensor element is required to have on the front side thereof, a lid as means for effecting the selective incidence of infrared radiation. In the meantime, the sensor element generally has on the reverse side thereof an opening of desired shape and size formed during the fabrication of the bridging structure so as to allow anisotropic etching such as, with an aqueous hydrazine solution. This opening is likewise preferably sealed with a lid to diminish the otherwise inevitable effect of reflection and absorption of infrared radiation through the opening.
To cope with the situation, the following measure has been conceived. Some hundreds of sensor elements are simultaneously incised in one silicon wafer and the sensor elements still held on the wafer are tested for electrical properties as with a probe to discriminat between conforming and non-conforming articles. Thereafter, the sensor elements are severally cut off the wafer with a dicing saw. Each of the separated sensor elements is then washed, with a lid joined to the front side (the side for admitting the incident infrared radiation) thereof and another lid joined to the rear side thereof.
This method, however has the following problems. For example, at the step of cutting the silicon wafer with the dicing saw, the practice of feeding cooling water to the dicing saw for the purpose of removing the heat generated between the chip and the rotary blade, lubricating the dicing saw, and ensuring chip ejection is prevalent. This cooling water causes breakage of the bridging structure. In this case, the breakage of the bridging structure can be precluded by precoating the chip with electron wax. Since the chip must be subsequently cleaned of this wax as by washing, the impact of this washing brings about breakage of the bridging structure.
In the conventional infrared sensor, the possible loss of the energy of the infrared radiation impinging on the infrared temperature-sensitive film is avoided by having this infrared temperature-sensitive film formed on the minute bridging structure as described above. Since the energy of the infrared radiation impinging on the infrared temperature-sensitive film is lost through the air which envelopes the infrared temperature-sensitive film, the problem arises that an infrared sensor which excels in practical utility and possesses high sensitivity cannot be obtained simply by imparting a bridging structure to the infrared temperature-sensitive film.
Two methods are available for the formation of the bridging structure. One of them comprises the steps of forming an excavated part in a sensor substrate, temporarily refilling the excavated part thereby forming a sacrificial layer, forming on this sacrificial layer a pattern for the bridging structure, forming an infrared temperature-sensitive film on the pattern, and then removing the sacrificial layer while leaving the formed bridging part behind. The other method comprises the steps of forming on one surface of a sensor substrate a pattern for a bridging part and an infrared temperature-sensitive film and performing an anisotropic etching on the other surface of the sensor substrate thereby forming a cavity below the bridging pattern and completing the bridging structure. The latter method is easier procedurally than the former method.
Incidentally, in the latter method, hydrazine or an aqueous potassium hydroxide solution is used to effect the anisotropic etching. Since this etchant is strongly alkaline, it dissolves not only the silicon as the material for the sensor substrate but also materials other than silicon such as, for example, aluminum which is generally used as the material for electrodes and conductors. Thus, the manufacture of the bridging structure needs to resort to the following method, for example.
This method comprises the steps of forming an insulating film intended to form a bridging part on a silicon substrate destined to form a sensor substrate, superposing an infrared temperature-sensitive film on the insulating film, forming thereon a wiring layer and an electrode pad both of aluminum, coating the entire surface with a protective film (insulating film), then carrying out the anisotropic etching selectively on the rear surface of the silicon substrate thereby forming a bridging part, subsequently boring a contact hole in the protective film on the upper surface of the silicon substrate to a depth reaching the electrode pad, and electrically connecting the electrode pad and a flexible printed substrate (FPC) by soldering through the medium of the contact hole.
An object of this invention, therefore, is to provide an infrared sensor which is furnished with a stable bridging structure and is small and inexpensive and a method for the production thereof.
Another object of this invention is to provide a method for the production of an infrared sensor which allows easy formation of a contact hole on an electrode pad without involving breakage of a bridging part.
A further object of this invention is to provide an infrared sensor such that during the process of etching for the formation of electrodes, the phenomenon of side etching if allowed to occur will produce no gap between electrodes and an insulating film or no dispersion in the magnitude of electrical resistance of an infrared temperature-sensitive film.
Yet another object of this invention is to provide an infrared sensor of high-sensitivity which enables the amount of energy lost through the periphery of the infrared temperature-sensitive film to be minimized.